heritage

Article Understanding Deterioration due to Salt and Ice Crystallization in Scandinavian Massive Brick Masonry

Kristin Balksten 1 and Paulien Strandberg-de Bruijn 2,*

1 Conservation, Uppsala University Campus Gotland, 621 67 Visby, Sweden; [email protected] 2 Division of Building Materials, Lund University, 221 00 Lund, Sweden * Correspondence: [email protected]; Tel.: +46-046-222-4260

Abstract: Extensive durability problems such as weathering and degradation are found in historic Scandinavian brick masonry buildings, especially from the neo-Gothic period. These are largely due to the crystallization of salts and frost action in the bricks and mortars. This article aims to show and illustrate which salts and crystals are found in historic brick masonry buildings and to describe their appearance and behavior. An additional aim is to explore possibilities of preventing salt-related damage on internal masonry wall surfaces, such as using hemp-lime sacrificial plaster beneath the plaster. The objective is to show the mechanisms behind salt-related problems and to perform a case study and a laboratory study on salt-damaged brick masonry containing sodium sulphate. In order to prevent and stop damage to the masonry, it is important to be able to identify the nature of the salt damage and the type of salt that caused the damage. Neo-Gothic brick masonry buildings require well-planned, continuous maintenance of the masonry. It is therefore of the utmost importance to have an understanding of the complex functions of the masonry and of the salts that can cause damage to these historic buildings.



 Keywords: brick masonry; lime mortar; salt deterioration; thenardite; mirabilite; halite; calcite; Citation: Balksten, K.; frost damage Strandberg-de Bruijn, P. Understanding Deterioration due to Salt and Ice Crystallization in Scandinavian Massive Brick Masonry. 1. Introduction Heritage 2021, 4, 349–370. https://doi.org/10.3390/ The main aim of this article is to show in a pedagogic way the most common salts heritage4010022 and crystals found in historic Scandinavian masonry buildings, to describe and illustrate how to use ocular methods to recognize them in macroscopic and microscopic scales Received: 29 January 2021 and to describe their behaviour when causing damage. It seeks to provide an in-depth Accepted: 8 February 2021 understanding of salt and frost-related damage and knowledge of how to repair and Published: 11 February 2021 renovate brick masonry walls without causing additional damage. It is aimed at those having to deal with damaged masonry in practice. Publisher’s Note: MDPI stays neutral An additional aim is to explore and present a possible new way to prevent or delay with regard to jurisdictional claims in salt-related damage on internal masonry wall surfaces. The objective was to perform a case published maps and institutional affil- study and full-scale laboratory studies on brick masonry walls with a hemp-lime plaster iations. and to study salt-induced precipitation from the brick wall into the hemp-lime plaster. Since the early 13th century, brick masonry has been used for construction in the Scandinavian countries [1], using different construction techniques and with the quality of materials varying over time. The differences have led to a variation in the behaviour of Copyright: © 2021 by the authors. these masonries in terms of weathering and degradation. Brick masonry buildings from the Licensee MDPI, Basel, Switzerland. neo-Gothic period and to some extent in the neo-Romanesque style have proven to be extra This article is an open access article problematic due to their construction method and choice of materials compared to, for distributed under the terms and example, medieval massive brick masonry buildings. It became clear at an early stage that conditions of the Creative Commons poor quality bricks and lime mortar with high porosity and poor frost resistance and bricks Attribution (CC BY) license (https:// containing sulphate had been used in the masonry core. The cause of moisture problems creativecommons.org/licenses/by/ in neo-Gothic buildings has been examined in more in-depth studies [2–6]. The types of 4.0/).

Heritage 2021, 4, 349–370. https://doi.org/10.3390/heritage4010022 https://www.mdpi.com/journal/heritage Heritage 2021, 4 350

damage that can be found in these masonries are very similar, due to a large extent on the crystallization of both salts and ice in the porous bricks and mortars. All the salts presented in this paper can be found in the neo-Gothic buildings studied but the photographs used may come from other historic buildings where they are more illustrative. The illustrations and good examples included here are derived from 20 years of practical work with historic masonry buildings and their decay. This paper, therefore, focuses primarily on salt and frost-related damage found in Scandinavian neo-Gothic masonry buildings based on their structure, function and degra- dation processes. Its purpose is to provide an understanding of what these salt and frost- related moisture problems entail: their appearance, what causes them and, where possible, measures to minimise their harmful effect on the durability of the masonry. Salt and frost problems in historic Scandinavian masonry buildings are often noted [2,7–12], but the complexity of these neo-Gothic brick walls mean that the damage is often very extensive and costly. This paper is based on previous international and national research, a case study and a laboratory test plus a number of field studies of historic buildings. There is essential international research focusing on the occurrence and behaviour of salts in porous building materials [13–27]. These provide in-depth knowledge of how different salts crystallise and cause damage. Microscopy is a widely accessible and relatively straightforward method to identify salts and to determine their occurrence and behaviour in porous building materials. A recent literature review on test methods presents laboratory tests that are often used [28]; very little is said about microscopic methods for identification. There is also valuable research presenting theoretical models, numerical simulations and quantitative studies on crystal growth in porous building materials in general [29–35] and in bricks and mortars in particular [36–40]. Several studies on frost damage in porous materials are of interest and have provided more in-depth knowledge of the influence of pore structure on durability [9,11,41–46]. Several papers combining numerical models and microscopic and/or macroscopic studies are of interest [44,47–50]. Some studies using microscopic methods on crystal growth are of particular interest for this paper, and previous research on the subject is of importance for understanding and interpretation [15,51–54]. The problem has also been studied continuously throughout the 20th century focusing on Nordic climatic conditions and brick masonries [7–12,55], and a great number of objects built around the year 1900 have been investigated with the aim of restoration [2–6]. The results presented are the results of the research project ‘Preventing salt-induced precipita- tion through hemp-lime plaster in older brick buildings’, funded by the Swedish Energy Agency through the “Spara och Bevara” research programme. The paper is divided into two main parts. The first part describes the Scandinavian neo- Gothic brick masonry buildings and the salt and frost-related damage found in them [2–6]. The second part describes a case study and laboratory study of the use of lime-hemp inside salt-damaged brick masonry buildings.

2. Neo-Gothic Brick Architecture in Sweden There are extensive salt problems in brick masonry in neo-Gothic buildings in Sweden. The south and west façades of high church towers have exhibited moisture and salt-related problems since the day they were built [2]. The construction of these neo-Gothic buildings was new and innovative in the 1870s, when cement and façade bricks simultaneously came into use in Sweden. Before this period, brick masonry was generally constructed using high-quality brick in homogeneous interacting monoliths in combination with high-quality lime mortar [56,57]. Neo-Gothic buildings were commonly constructed using bricks with air lime mortar (lime:sand ratio 1:2 to 1:3), as described by architects in the building instructions preserved in the archives as well as in literature from the 19th century [58]. It became clear at an early stage that both bricks and lime mortar of poor quality had been used in the masonry core Heritage 2021, 4 351

of these buildings. In contrast to the masonry core, the façade was built using hard-burned bricks with a dense surface combined with a thin layer of rigid, dense, cement-rich pointing mortar. Among the bricks in the core there were usually some stones of lower quality that were relatively unburned, of sulphur-containing clay. The sulphur has given rise to salt-related damage in the masonry. Additionally, the façade stones sometimes lacked sufficient anchoring in the masonry core wall, since anchoring was accomplished by means of a header course that was tied to the core of the wall. Sometimes only every seventh course was a header course that was anchored to the masonry behind. This has caused structural problems at times, as the façade brick sometimes became detached from the masonry core. This is evident from the study of archives and pilot studies before restoration on many of those churches [2–6]. When the pointing is stiff and dense and the brick surface is hard and dense there is a risk of crack formation between bricks and joints as well as parallel to the surface layer of the brick, caused by temperature and moisture movements. With façade brick surfaces detaching from the masonry core and with cracks between bricks and joints, water can enter into the masonry by capillary action, leading to moisture accumulation in the masonry core. This, in turn, contributes to frost damage in the surface, dissolution of lime in the core and salt transports with salt crystallization on internal and external wall surfaces. With salts abundantly present in the masonry, salt problems have emerged early, with salt damage internally on the plaster and externally causing brick surfaces, renders and joints to spall off.

3. Finding, Recognizing and Understanding Salt and Ice Deformation in Brick Masonry It has been known for a long time that the presence of salts can cause damage to masonry. Several ways to avoid salts in brick masonry were documented in the 19th century. Among other things, it is imperative to avoid the inclusion of sand that may contain salts in the mortar [59]. It has also been known that salts of sulphur can be formed in the brick production process if the clay contains pyrite [55,58,60]. Salt weathering can occur in two different ways: chlorides (e.g., from sea salts or food storage) deposit a white beard of salts that grows outwards from the surface (efflorescence), while the salts from the masonry cause material loss since they grow inside a porous media and cause weathering from within (subflorescence). If there is a combination of salts, they will interact and cause damage [61,62]. This paper presents the salts one by one to aid understanding. Frost can also often be a contributing factor in degradation and the damage can be reminiscent of salt weathering (although no crystals remain to be observed at temperatures above zero). Sodium sulphate can occur in bricks if they are slightly underburnt. This happens when sulphate reacts with combustion gases forming sulphuric acid [55,63]. If the clay contains pyrite, there is a known risk of the brick containing masonry salts. In the case of proper combustion, all sulphur in the sulphuric acid (FeS2) should be expelled, but sometimes the pyrite is only converted to iron sulphide (FeS) which in itself can contribute to water absorption and expansion [55]. Calcium and carbonates are naturally found in all air lime mortar that occurs in masonry. If there is a presence of acidic salts, the solubility of calcium carbonate increases due to acid-base reactions. Calcium sulphate (i.e., gypsum) can be used as an additive in cement but it can also be formed on lime-based surfaces when there is sulphur in the air, e.g., from car exhaust fumes [64,65]. Sodium chloride is commonly found in the sea, which is why these salts are often found in sea-facing masonry. Previous use of the building for food storage may also cause salt occurrence in the form of chlorides. Sodium chloride and calcium chloride can also occur in masonry as a result of the use of road salts, for example on stairs and pavements [13]. In order to give a greater understanding of the specific problems that most commonly occur in brick masonry, we provide an in-depth study of the phenomena that can be Heritage 2021, 4 FOR PEER REVIEW 4

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also cause salt occurrence in the form of chlorides. Sodium chloride and calcium chloride can also occur in masonry as a result of the use of road salts, for example on stairs and also cause salt occurrence in the form of chlorides. Sodium chloride and calcium chloride Heritage 2021, 4 pavements [13]. 352 can also occur in masonry as a result of the use of road salts, for example on stairs and In order to give a greater understanding of the specific problems that most com- pavements [13]. monly occur in brick masonry, we provide an in-depth study of the phenomena that can In order to give a greater understanding of the specific problems that most com- be observed. This involves a deeper understanding of chloride and sodium sulphate and observed.monly occur This in involves brick masonry, a deeper we understanding provide an in- ofdepth chloride study and of the sodium phenomena sulphate that and can specific problems that are linked to these particular salts. We also discuss the appearance specificbe observed. problems This that involves are linked a deeper to these understanding particular salts. of Wechloride also discuss and sodium the appearance sulphate ofand of gypsum clusters and the solubility of calcium carbonate and what happens when cal- gypsumspecific clusters problems and that the are solubility linked ofto calciumthese particular carbonate salts. and We what also happens discuss when the appearance calcium cium hydroxide and calcium carbonate are exposed to water for a long time. In the ob- hydroxideof gypsum and clusters calcium and carbonate the solubility are exposed of calcium to watercarbonate for aand long what time. happens In the when objects cal- jects studied in Scandinavia, the following salts and crystals are those most represented studiedcium hydroxide in Scandinavia, and calcium the following carbonate salts are and exposed crystals to are water those for most a long represented time. In the to aob- to a greater or lesser extent. We also use photographs of other objects with the aim of greaterjects studied or lesser in extent. Scandinavia, We also the use following photographs salts of and other crystals objects are with those the aimmost of represented showing showing illustrative and pedagogic examples for each crystallization process. illustrativeto a greater and or pedagogic lesser extent. examples We also for eachuse p crystallizationhotographs of process. other objects with the aim of showing illustrative and pedagogic examples for each crystallization process. 3.1. Sodium3.1. Sulphate Sodium Sulphate 3.1.The Sodium most Sulphate harmfulThe most salts harmful for a plaster salts orfor a painteda plaster wall or appeara painted to be wall the sulphatesappear to [15be, 66the], sulphates [15,66], see Figures 1–4. These can be based on sodium Na2SO4 (forming salt crystals see FiguresThe most1–4. Theseharmful can salts be for based a plaster on sodium or a painted Na 2SO 4wall(forming appear salt to be crystals the sulphates called thenarditecalled or mirabilite) thenardite but or also mirabilite) potassium but KalsoSO potassium, magnesium K2SO MgSO4, magnesiumand calcium MgSO4 and cal- [15,66], see Figures 1–4. These can be based 2 on sodium4 Na2SO4 (forming4 salt crystals CaSO4 [67].cium These CaSO are4 particularly [67]. These harmfulare particularly because harmful sulphates because crystallise sulphates in places crystallise where in places called thenardite or mirabilite) but also potassium K2SO4, magnesium MgSO4 and cal- they causewhere great damage. they cause They great can damage. cause disintegration They can cause due disintegration to crystallization due in to a c porousrystallization in a cium CaSO4 [67]. These are particularly harmful because sulphates crystallise in places material andporous they canmaterial also affect and thethey dissolution can also affect of lime the due dissolution to acid-base of reactions.lime due Sodiumto acid-base reac- where they cause great damage. They can cause disintegration due to crystallization in a sulphate cantions. crystallise Sodium inside sulphate the can surface crystallise and switch inside between the surface two and phases switch (Na betweenSO and two phases porous material and they can also affect the dissolution of lime due to acid2-base4 reac- Na SO ·10H(NaO)2SO with4 and an Na explosive2SO410H effect2O) with due anto hydration explosive pressure effect due and to crystallization hydration pressure and tions.2 4 Sodium2 sulphate can crystallise inside the surface and switch between two phases pressure [15crystallizatio,66]. n pressure [15,66]. (Na2SO4 and Na2SO410H2O) with an explosive effect due to hydration pressure and crystallization pressure [15,66].

(a) (b) Figure 1. SodiumFigure sulphate 1. Sodium( aphotographed) sulphate in photographed a polarisation inmicroscope: a polarisation( b(a)) Sodium microscope: sulphate (a) Sodiumis dissolved sulphate and recrystal- is lized on the surface; (b) the sodium sulphate crystals grow in tree-like formations from the surface. Figure 1. Sodium sulphatedissolved photographed and recrystallized in a polarisation on the microscope: surface; ( b(a)) theSodium sodium sulphate sulphate is dissolved crystals growand recrystal- in tree-like lized on the surface; (b) theformations sodium sulphate from the crystals surface. grow in tree-like formations from the surface.

(a) (b)

(a) (b)

Figure 2. Sulphates are recognized by the fact that they often grow just below the surface: (a) A surface painted with impermeable paint. When the surface layer is lifted it falls off and leaves a slightly hollow wall surface, covered with white salt crystals; (b) the permeable lime wash is dissolved, leaving only superficial damage on the surface.

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Figure 2. Sulphates are recognized by the fact that they often grow just below the surface: (a) A surface painted with HeritageFigure2021, 4 2. SulphatesFigure are recognized2. Sulphates by are the recognized fact that they by theoften fact grow that justthey below often thegrow surface: just below (a) A the surface surface: painted (a) A with surface 353 painted with impermeable paint. When the surface layer is lifted it falls off and leaves a slightly hollow wall surface, covered with impermeable paint.impermeable When the surfacepaint. When layer theis lifted surface it falls layer off is and lifted leaves it falls a slightlyoff and hollowleaves awall slightly surface, hollow covered wall withsurface, covered with white salt crystals; (b) the permeable lime wash is dissolved, leaving only superficial damage on the surface. white salt crystals;white (b) the salt permeable crystals; (limeb) the wash permeable is dissolved, lime wash leaving is dissolved, only superficial leaving damage only superficial on the surface. damage on the surface.

(a) (b) (a) (a) (b) (b) Figure 3. Crystallised sulphates: (a) Newly crystallised sulphates give the illusion of being soft and fuzzy like cotton; (b) Figure 3. CrystallisedFigure sulphates: 3. FigureCrystallised (a 3.) NewlyCrystallised sulphates: crystallised sulphates: (a) Newly sulphate (a crystallised) Newlys give crystallisedthe sulphate illusion ssulphatesof give being the soft giveillusion and the fuzzyof illusion being like ofsoft cotton; being and softfuzzy(b) and like cotton; (b) newly formed crystals can be observed as tree-like crystal formations. newly formed crystalsnewly can formed fuzzybe observed crystals like cotton; as can tree (beb-)like observed newly crystal formed as formations. tree crystals-like crystal can be formations. observed as tree-like crystal formations.

(a) (b) (a) (a) (b) (b)

Figure 4. Sulphate damage to brick and cement surfaces: (a) Test wall built with bricks that were saturated with sodium Figure 4. SulphateFigure damage 4. Figure Sulphateto brick 4. and Sulphatedamage cement to damage bricksurfaces: and to brick(cementa) Test and wallsurfaces: cement built ( surfaces:witha) Test bricks wall (a ) thatbuilt Test were wallwith saturated builtbricks with that with bricks were sodium saturated that were with sodium sulphate. The surface of the bricks has been gradually degraded due to the explosive effect of the salts; (b) on a strong and sulphate. The surfacesulphate. of thesaturated Thebricks surface has with been of the sodium gradually bricks sulphate. has degraded been The gradually due surface to the degraded of explosive the bricks due effect hasto the been of explosive the gradually salts; (effectb) degradedon ofa strongthe salts; due and to(b ) the on a strong and dense cement plaster, sulphates are visible as dark salt roses where the paint layer has come off and the plaster has dense cement plaster,dense sulphates cementexplosive plaster, are effect visible sulphates of as the dark salts; are salt ( visibleb ) roses on a as strong where dark and thesalt dense paint roses layercement where has the plaster, come paint sulphates off layer and has the are come plaster visible off has as and dark the plaster has darkened as if it were constantly wet. darkened as if it weredarkened constantlysalt as if roses it wet. were where constantly the paint wet. layer has come off and the plaster has darkened as if it were constantly wet. 3.2. Black Crust from Gypsum 3.2.3.2. BlackBlack CrustCrust fromfrom3.2. Gypsum BlackGypsum Crust from Gypsum Calcium sulphateCalcium (gypsum) sulphate CaSO (gypsum)·2H O isCaSO usually3∙2H formed2O is usually on the surfaceformed ofon limestone the surface of lime- Calcium sulphateCalcium (gypsum) sulphate CaSO3 (gypsum)3∙2H2 2O is usuallyCaSO3∙2H formed2O is usuallyon the surfaceformed ofon lime- the surface of lime- stone or lime mortar as sulphur in the air reacts with lime to form gypsum [64,65]. Its salt orstone lime or mortar lime mortar asstone sulphur as or sulphur lime in the mortar air in reactsthe as air sulphur with reacts lime within tothe formlime air reactsgypsumto form with gypsum [64 lime,65]. Itsto [64,65] form salt crystals. gypsumIts salt [64,65]. Its salt crystals are called ettringite; see Figure 5. Where parts of the surface layer dissolve, it arecrystals called are ettringite; calledcrystals ettringite; see Figureare called see5. WhereFigu ettringite;re parts5. Where see of theFigu parts surfacere 5.of Wherethe layer surface dissolve,parts layer of the it dissolve, grows surface to itlayer dissolve, it grows to form an often dirty or black crust on the outside; see Figure 6. formgrows an to often form dirty angrows often or black to dirty form crust or an black onoften the crust dirty outside; on or the black see outside; Figure crust 6see on. theFigure outside; 6. see Figure 6.

Figure 5. A characteristic gypsum cross as photographed in a polarisation microscope.

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Figure 5. FigureA characteristic 5. A characteristic gypsum crossgypsum as photographed cross as photographed in a polarisation in a polarisation microscope. microscope.

(a) (a) (b) (b)

Figure 6. a Figure 6.( Figure()a Black) Black 6. crust (crusta) Black is usuallyis usually crust seen is seen usually when when sulphur seen sulphur when has reactedhassulphur reacted with has with limereacted onlime the with on surface thelime surface ofon calcareous the of surface calcareous rock of orcalcareous rock lime renderor lime rock or lime andrender lime and wash;render lime ( b andwash;) a close-uplime (b )wash; a close of a(b-up coloured) a ofclose a coloured-up lime of cementa colouredlime cement mortar lime mortar where cement thewhere mortar surface the where surface has transformedthe has surface transformed has into transformed gypsum. into gypsum. This into gypsum. causesThis causes theThis surface the causes surface to appear the to surface appear to be to veryto appear be dirty. very to Thedirt be y. originalvery The dirt original coloury. The colour oforiginal the limeof colourthe cement lime of cement the mortar lime mortar cancement still can mortar be still observed. be can observed. still be observed.

CalciumCalcium sulphateCalcium sulphate forms sulphate forms on ona forms rain-protected a rain on-protected a rain surface-protected surface or crystallises orsurface crystallises insideor crystallises a inside lime mortar a inside lime a lime bymortar the sulphating bymortar the sulphating by of the calcium sulphating of carbonate, calcium of calciumcarbonate, forming carbonate, needles forming in forming theneedles voids. needlesin Calcium the voids. in sulphatethe Calcium voids. is Calcium oftensulphate formedsulphate is often when formedis air often pollution whenformed air containing when pollution air pollution sulphur containing is containing involved sulphur in sulphuris the involved process is involved in and the isprocess mostin the process intenselyand is mostand visible intensely is most on the intensely visible surfaces on visible thatthe surfaces rain on the does surfacesthat not rain reach that does [64 rain ,not65 ];does reach see Figurenot [64,65] reach7.; see[64,65] Figure; see 7. Figure 7.

FigureFigure 7. 7.Stockholm FStockholmigure 7. Stockholm Castle Castle showing showing Castle a a colouredshowing coloured limea colouredlime cement cement lime render. render. cement As As sulphur-containing render. sulphur As- containingsulphur air-containing causesair air causes the formation of gypsum, this usually shows clearly where the rain impinges on a façade. As the formationcauses of gypsum, the formation this usually of gypsum, shows this clearly usually where shows the rain clearly impinges where on the a rain façade. impinges As gypsum on a façade. As gypsum isgypsum highly solubleis highly in soluble water, inthe water, damage the is damage greatest is on greatest surfaces on shelteredsurfaces shelteredfrom rain. from rain. is highly soluble in water, the damage is greatest on surfaces sheltered from rain.

3.3.3.3.Chlorides Chlorides3.3. Chlorides SodiumSodium chlorideSchlorideodium NaClchlorideNaCl usuallyusually NaCl deposits depositsusually ondepositson thethe surface, surface, on the formingsurface,forming salt formingsalt crystals crystals salt called crystalscalled called halite;halite; seeseehalite; FigureFigure see8 .8. ItFigure It grows grows 8. likeIt like grows a a beard, beard, like with awith beard, salt salt crystalswith crystals salt and andcrystals long long strandsand strands long [ 26[26]strands].. If If there there [26]. If there areare tight/impermeabletight/impermeableare tight/impermeable paint layers, paint itit layers, isis depositeddeposited it is deposited underunder thethe under surfacesurface the and andsurface liftslifts theandthe paint liftspaint the paint off,off, butbut ififoff, there there but is isif a athere homogeneous homogeneous is a homogeneous porous porous material, material,porous material, it it is is deposited deposited it is deposited on on the the surface surface on the [ 10 [10]surface].. If If [10]. If thethe sodium sodiumthe chloride chloridesodium comes chloridecomes from from comes sea sea from winds, winds, sea it it winds, does does not not it usuallydoes usually not cause causeusually great great cause damage. damage. great Indamage. In a a In a hydrationhydrationhydration oror dehydrationdehydration or dehydration process,process, itprocess,it may,may, however, however,it may, however, generategenerate generate dissolutiondissolution dissolution ofof thethe surface,surface, of the surface, sincesince atat differentsincedifferent at different temperaturestemperatures temperatures itit cancan alsoalso it affectcanaffect also otherother affect saltssalts other thatthat salts maymay that bebe presentpresentmay be [[62,67]present62,67].. [62,67]. SodiumSodium chlorides Sodiumchlorides chlorides are are often often foundare found often in in cellarsfound cellars usedin used cellars for for food usedfood storage; forstorage; food see storage;see examples examples see in examples Figurein Figure9. in Figure 9. 9.

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(a) (b) (a) (a) (b) (b)

FigureFigureFigure 8. (a ) 8.8. Halite ((aa)) HaliteHalite crystals crystalscrystals recrystallised recrystallisedrecrystallised and photographedandand photographedphotographed in a polarisationinin aa polarisationpolarisation microscope; microscope;microscope; (b) a (( macrobb)) aa macromacro photo photophoto of compact ofof compactcompact Figure 8. (a) Halite crystals recrystallised and photographed in a polarisation microscope; (b) a macro photo of compact halitehalitehalite crystals. crystals.crystals. halite crystals.

((aa)) (b) (a) (b) (b) Figure 9. a FigureFigureExamples 9.9. ExamplesExamples of halite ofof halitehalite crystals crystalscrystals that formedthatthat formedformed in basements inin basementsbasements where wherewhere food foodfood had beenhadhad beenbeen stored: stored:stored: ( ) Halite ((aa)) HaliteHalite crystals crystalscrystals on a on brickon aa brickbrick Figure 9. Examples of halite crystals that formed in basements where food had been stored: (a) Halite crystals on a brick wall;wall;wall; (b) halite ((bb)) halitehalite crystals crystalscrystals grow growgrow from fromfrom a surface, aa surface,surface, forming formingforming both both compactboth compactcompact crystals crystalscrystals and long andand strands.longlong strands.strands. wall; (b) halite crystals grow from a surface, forming both compact crystals and long strands. 3.4. Calcium Carbonates 3.4.3.4. 3.4.CalciumCalcium Calcium CarbonatesCarbonates Carbonates CCalciumalcium carbonatecarbonate CaCOCaCO33 isis thethe binderbinder inin limelime mortarmortar andand limestone.limestone. CalciumCalcium car-car- CalciumCalcium carbonatecarbonate CaCO 3 isis the the binder binder in in lime lime mortar mortar and and limestone. limestone. Calcium Calcium car- bonate itself has no disintegrating effect when crystallised [67,68], forming salt crystals carbonatebonatebonate itself itself itself has has has no no disintegratingno disintegrating disintegrating effect effect effect when when when crystallis crystallised crystallised [67,68] [ed67 ,[67,68]68],, formingforming, forming saltsalt salt crystalscrystals crystals called calcite; see Figure 10. It can, however, dissolve and recrystallise under prolonged calledcalledcalled calcite;calcite; calcite; seesee FigureseeFigure Figure 10 10.. It1It0 can,.can, It can, however, however, however, dissolve dissolve dissolve and and and recrystallise recrystallise recrystallise under under under prolonged prolonged prolonged humid conditions and with the presence of acidic salts. Upon recrystallization, it forms humidhumidhumid conditionsconditions conditions andand and withwith with thethe presencethepresence presence ofof acidic acidicof acidic salts. salt salts. UponUpons. Upon recrystallization,rec rystallizatiorecrystallization, ititn, formsforms it forms crusts that are very hard and difficult to dissolve. These can sometimes be seen on the crustscrustscrusts thatthat that areare veryarevery very hardhard hard andand and difficultdifficult difficult to to dissolve. dissolve.to dissolve. TheseThese These cancan can sometimessometimes sometimes bebe seen seenbe seen onon thetheon the outside of masonry repointed with cement, in the form of floatstones and stalactites; see outsideoutsideoutside ofof masonrymasonry of masonry repointedrepointed repointed withwith with cement,cement, cement, inin thethe in formtheform form ofof floatstonesfloatstones of floatstones andand and stalactites; stalactites; stalactites; seesee see Figures 11 and 12. It can also sometimes be seen as a thick, colour- intensive layer on lime FiguresFiguresFigures 11 11 and and 11 and12 12.. ItIt12. cancan It can alsoalso also sometimessometimes sometimes bebe seenseen be seen asas aa as thick,thick, a thick, colour-colour colour- intensive intensive- intensive layer layer layer on on lime limeon lime render that has a dense film of binder at the surface, where the lime from inside the renderrenderrender that that hasthat has a has dense a dense a dense film film of film binder of binder of atbinder the at surface, the at the surface, wheresurface, where the where lime the from the lime lime inside from from the inside render inside the the render has been deposited on the surface in a glassy calcite layer on the external surface hasrenderrender been has deposited hasbeen been deposited ondeposited the surfaceon theon thesurface in asurface glassy in a in calciteglassy a glassy layercalcite calcite on layer the layer externalon theon theexternal surface external surface of thesurface of the lime wash; see Figures 13 and 14. limeof theof wash; thelime lime seewash; Figureswash; see seeFigure 13 Figureands 1314s .and 13 and 14. 14.

FigureFigure 10.10. PolarisationPolarisation photographphotograph showingshowing aa glassyglassy calcitecalcite crystal.crystal. UsuallyUsually thesethese areare veryvery small,small, Figure 10. Polarisation photograph showing a glassy calcite crystal. Usually these are very small, Figuremakingmaking 10. Polarisation itit difficultdifficult toto photograph identifyidentify themthem showing throughthrough a glassy aa polarisationpolarisation calcite crystal. microscope.microscope. Usually these are very small, makingmaking itit difficultdifficult to to identify identify them them through through a a polarisation polarisation microscope. microscope.

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(a)( a) (a) (b()b ) (b)

Figure 11. When lime mortar is trapped in a damp wall that is exposed to water for a long time, the lime dissolves: (a) The FigureFigure 11. 11. When When lime Figurelime mortar mortar 11. Whenis istrapped trapped lime in mortar ina dampa damp is trappedwall wall th atth in atis a isexposed damp exposed wall to towater th waterat is for exposed for a longa long totime, time,water the the forlime lime a longdissolves: dissolves: time, the(a )( a Thelime) The dissolves: (a) The mortarmortar inmortar the brickin inthe the wall brick brick is mortarwall dissolved wall is isdissolvedin dissolvedthe behind brick behind the wallbehind dense is the dissolved the surfacedense dense surface ofbehind surface façade of the offaçade brick façadedense and brick surface brick cement and and of cement mortar; façadecement mortar; brick ( bmortar;) lime and (b mortar() b cementlime) lime mortar from mortarmortar; the from from (b )the lime the mortar from the inside ofinsideinside the wallof ofthe isthe wall deposited wall isinside isdeposited deposited as of floatstone the as wall asfloatstone floatstone onis deposited the surface.on on the theas surface. floatstone surface. on the surface.

(a)( a) (a) (b()b ) (b)

FigureFigure 12. 12. Lime Lime dissolved dissolvedFigureFigure 12. inside 12.inside LimeLime masonry dissolvedmasonry dissolved repointed repointedinside inside masonry with masonry with cement cement repointed repointed is isdeposited deposited with with cement as cement asstalactites stalactites is isdeposited deposited on on the theas surface: asstalactites surface: stalactites (a )( aStalactiteson) Stalactites on the the surface: (a) Stalactites fromfrom dissolved dissolved lime limefrom mortarsurface: mortar dissolved on ( aon )Visby Stalactites Visby lime city mortarcity wall; from wall; on(b dissolved () Visbyb floatstone) floatstone city lime wall; on mortaron a granite(ab granite) floatstone on Visbywall. wall. cityon a wall; granite (b) floatstonewall. on a granite wall.

(a)( a) (a) (b()b ) (b)

Figure 13. Calcite crystals: (a) Calcite crystals have grown slowly over time on a surface with free access to calcium FigureFigure 13. 13. Calcite Calcite crystals:Figure crystals: 13. (a )(Calcite aCalcite) Calcite crystals: crystals crystals (havea )have Calcite grown grown crystals slowly slowly have over over grown time time on slowly on a surfacea surface over with time with freeon free a access surface access to towithcalcium calcium free hy- access hy- to calcium hy- hydroxidedroxidedroxide and water.and and water. water. Thedroxide largeThe The large trigonal large and trigonal water.trigonal crystals crystalsThe crystals are large approx. are trigonalare approx. approx. 5 mm; crystals 5 (mm;b5 )mm; a surfaceare(b () bapprox.a) surfacea wheresurface 5 wheremm; calcite where ( bcalcite crystals) calcitea surface crystals werecrystals where formed were were calcite formed on formed thecrystals on on the werethe formed on the externalexternal surfaceexternal surface of surface a matt of limeofexternala matta matt paint. lime surface lime This paint. paint. givesof Thisa Thismatt an gives apparently giveslime an paint.an apparently apparently oily This film gives oily with oily anfilm a film glassyapparently with with character.a glassya glassyoily character.film The character. hexagonalwith aThe glassyThe hexagonal shapes hexagonal character. of theshapes shapes The ofhexagonal of shapes of crystalsthe arethe crystals discernible. crystals are are discernible. discernible.the crystals are discernible.

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(a) (b) (a)( a) (b)( b)

Figure 14. CalciteFigure crystallization14. Calcite crystallizatio and recrystallization:n and recrystallizatio (a) A thick,n: (a) glassy A thick, film glassy of calcite film of on calcite the external on the surfaceexternal of surface a lime of a lime FigureFigure 14. 14. Calcite Calcite crystallizatio crystallization andn and rec recrystallizatiorystallization: n:(a )( aA) Athick, thick, glassy glassy film film of ofcalcite calcite on on the the external external surface surface of ofa limea lime wash. The calcitewash. The film calcite prevents film new prevents lime washnew lime from wash attaching, from attaching, causing the causing lime washthe lime to spall wash off to whenspall off freezing; when freezing; (b) the (b) the wash.wash. The The calcite calcite film film prevents prevents new new lime lime wash wash from from attaching, attaching, causing causing the the lime lime wash wash to tospall spall off off when when freezing; freezing; (b )( bthe) the surface of asurface porous of lime a porous render lime restored render in restored the 1970s in has the changed1970s has colour changed due colour to some due recrystallization. to some recrystallization. surfacesurface of ofa porous a porous lime lime render render restored restored in inthe the 1970s 1970s has has changed changed colour colour due due to tosome some rec recrystallizatiorystallization. n. 3.5. Ice Formation 3.5. Ice3.5.3.5. Formation Ice Ice Formation Formation Water forms ice crystals, expanding in volume by about 10% and breaking up wa- WWaterater forms forms ice ice crystals, crystals, expanding expanding in involume volume by by about about 10 10% %and and breaking breaking up up wa- wa- Waterter forms-saturated ice crystals, porous expandingmaterials [45] in. volume Inside unheated by about (church) 10% and towers, breaking it upis not water- uncommon terter-saturated-saturated porous porous materials materials [45] [45]. Inside. Inside unheated unheated (church) (church) towers, towers, it isit isnot not uncommon uncommon saturatedto porous find thick materials layers [of45 ].ice Inside crystals unheated as they form (church) quite towers, slowly iton is a not water uncommon-saturated to masonry to find thick layers of ice crystals as they form quite slowly on a water-saturated masonry find thickto findsurface; layers thick ofsee layers ice Figures crystals of ice 15 crystals asand they 16. formasThe they external quite form slowly quitewall surfaceslowly on a water-saturated onmay a wateralso bear-saturated witness masonry masonry to the ef- surface; see Figures 15 and 16. The external wall surface may also bear witness to the ef- surface;surface; seefect Figures of see frost Figures 15damage and 15 16 andsuch. The 16. as externalspalling/flakingThe external wall wall surface of surfacethe maymortar. may also In also bear renders, bear witness witness damage to theto can the occur ef- fect of frost damage such as spalling/flaking of the mortar. In renders, damage can occur effect offect frostbetween of frost damage damagelayers such of assuch render spalling/flaking as spalling/flakingor between of the the oflime mortar. the wash mortar. In and renders, In the renders, render; damage damage see can Figure occur can occur17. On between layers of render or between the lime wash and the render; see Figure 17. On betweenbetween layersbrick, oficelayers render crystals of orrender can between cause or between thedamage lime washthecausing lime and parts wash the render;of and the brickthe see render; Figure to spall 17see off.. On Figure brick, 17. On ice crystalsbrick,brick, ice can ice crystals causecrystals damagecan can cause cause causing damage damage parts causing causing of the parts brick parts of to ofthe spall the brick brick off. to tospall spall off. off.

(a) (b) (a)( a) (b)( b) Figure 15. Frost damage on masonry: (a) A layer of ice covering brick inside a church tower. The ice is formed from wa- FigureFigureFigure 15. terFrost 15.- 15.saturated Frost damageFrost damage damage brick on masonry:onand on masonry: so masonry: this (a effect) ( Aa )( layeraA )is layerA not layer of found iceof ofice covering ice oncovering coveringthe adjacent brick brick brick inside insidegranite; inside a churcha church (ab )church frost tower. tower.-damaged tower. The The The ice brick.ice ice is is formed formedisOn formed some fromfrom bricks,from wa- wa- only water-saturatedterter-saturated-saturatedthe compact brick brick brick and surfaceand soand so this solayerthis effectthis effect has effect is frozen notis notis found not off,found found and on theon on on the adjacentothers the adjacent adjacent deeper granite; granite; frostgranite; (b )damage frost-damaged(b )( bfrost) frost has-damaged -resulteddamaged brick. brick.in Onbrick. more someOn extensiveOn some bricks, some bricks, materialbricks, only only only loss. the compactthethe compact compact surface surface surface layer layer has layer frozen has has frozen off,frozen and off, off, onand othersand on on others deeperothers deeper frostdeeper damagefrost frost damage damage has resulted has has resulted resulted in more in inmore extensive more extensive extensive material material material loss. loss. loss.

(a) (b) (a)( a) (b)( b) Figure 16. Ice crystal formation on masonry: (a) Ice crystals cover the entire inside of a brick tower on a cold winter’s day. Figure 16. IceHere, crystal the ice formation crystals have on masonry: formed an (a) armour Ice crystals-like layer cover of the ice entire as well inside as some of a brickfiliform tower crystals on a of cold approx. winter’s 2 cm; (b) an FigureFigure 16. 16. Ice Ice crystal crystal formation formation on on masonry: masonry: (a )( aIce) Ice crystals crystals cover cover the the entire entire inside inside of ofa brick a brick tower tower on on a cold a cold winter winter’s day.’s day. day. Here, the ice crystals have formed an armour-like layer of ice as well as some filiform crystals of approx. 2 cm; (b) an Here,Here, the the ice ice crysta crystals havels have formed formed an an armour armour-like-like layer layer of ofice ice as aswell well as assome some filiform filiform crystals crystals of ofapprox. approx. 2 cm;2 cm; (b )( ban) an example from a basement where a thick layer of porous ice crystals covers the internal concrete surface. This phenomenon can be observed when porous materials saturated with water are exposed to frost.

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example from a basement where a thick layer of porous ice crystals covers the internal concrete surface. This phenome- non can be observed when porous materials saturated with water are exposed to frost.

(a) (b)

FigureFigure 17. Spalling17. Spalling of lime of lime mortar mortar due todue ice to formation: ice formation: (a) When (a) When lime mortarlime mortar freezes freezes and spalls and spalls off it leaves off it leaves a characteristic a character- flake-shapedistic flake- pattern;shaped (pattern;b) each separate(b) each renderseparate layer render spalled layer off spalled is caused off byis caused freezing by temperatures freezing temperatures in a lean lime in a render lean lime saturatedrender with saturated water. with water.

3.6. The Behaviour of Different Salts That Cause Degradation 3.6. The Behaviour of Different Salts That Cause Degradation Due to the way different salts behave, they can often be identified based on how Due to the way different salts behave, they can often be identified based on how they are deposited, the type of damage they have caused and how they crystallise. The they are deposited, the type of damage they have caused and how they crystallise. The appearance of some salt crystals is very characteristic provided they grow from a con- appearance of some salt crystals is very characteristic provided they grow from a concen- centrated saline. These are often easy to recognize both in the field and through a polar- trated saline. These are often easy to recognize both in the field and through a polarisation isation microscope [12,69]; see photographs on the previous pages. microscope [12,69]; see photographs on the previous pages. DifferentDifferent salts salts act inact different in different ways. ways. Depending Depending on the on type the of type salt presentof salt present in a masonry, in a ma- theysonry, differ they greatly differ in greatly behaviour in behaviour in relation in torelation factors to such factors as temperaturesuch as temperature and relative and rel- humidityative humidity (RH) [19 ,(RH)68]. For [19,68] example,. For example, each salt haseach a salt specific has criticala specific RH critical for crystallization; RH for crystal- seelizatio Table1n;. This see Table RH can 1. alsoThis be RH called can also equilibrium be called RH, equilibrium and it is the RH climate, and it that is the a saturated climate that salta solution saturated gives salt risesolution to in, gives for example, rise to in, a closedfor example, climate a chamberclosed climate [23]. chamber [23].

Table 1. CommonTable salts 1.in masonry,Common their salts critical in masonry, Relative their Humidity critical ( RelativeRH), solubility Humidity and molar (RH), solubilitymass [67,68] and. molar Chemicalmass Formu- [67,68]. Critical RH for Crystallization at 20 °C Solubility Name Molar Mass [g/molar] la [%] Critical RH for [g/L] Chemical Molar Mass halite NaCl Name 75 Crystallization Solubility360 [g/L] 58.5 Formula ◦ [g/molar] thenardite Na2SO4 82 at 20 C [%] 162 142 mirabilite Na2SO4·10Hhalite2O NaCl91 75900 360 58.5322 gypsum/ettringite CaSO3·2Hthenardite2O Na2SO4 ~100 822 162.4 142172 mirabilite Na SO ·10H O 91 900 322 lime/calcite CaCO3 2 4 ~1002 14·× 10−3 100 gypsum/ettringite CaSO3·2H2O ~100 2.4 172 Salts react in different ways when they are exposed to water.−3 Salts are readily solu- lime/calcite CaCO3 ~100 14 · 10 100 ble in water, so that when there are enough water molecules surrounding a salt molecule, a salt solution is formed. If the salt concentration is saturated, a salt solution is formed, Salts react in different ways when they are exposed to water. Salts are readily soluble otherwise a dilute salt solution [67,68] is formed. In porous materials, the presence of in water, so that when there are enough water molecules surrounding a salt molecule, hygroscopic bound water is sufficient for salts to dissolve. The relative humidity together a salt solution is formed. If the salt concentration is saturated, a salt solution is formed, with access to free water affects this process. For crystals that have a critical RH of ~100%, otherwise a dilute salt solution [67,68] is formed. In porous materials, the presence of i.e., lime/calcite, access to free water is generally required over a long period to enable hygroscopic bound water is sufficient for salts to dissolve. The relative humidity together them to dissolve. with access to free water affects this process. For crystals that have a critical RH of ~100%, Different salts grow in different ways, and they each have their own distinct deg- i.e., lime/calcite, access to free water is generally required over a long period to enable radation mechanism [23]; see illustrations in Figure 18. Halite grows from the surface in a them to dissolve. filiform shape, while mirabilite/thenardite grows inside the material forming a tree-like Different salts grow in different ways, and they each have their own distinct degra- pattern and causing the material to fall apart. Calcite crystals form a hard, glassy shell on dation mechanism [23]; see illustrations in Figure 18. Halite grows from the surface in a the external surface of a material, while ettringite forms crusts where lime is dissolved filiform shape, while mirabilite/thenardite grows inside the material forming a tree-like patternand andintegrated. causing The the materialdrawings to of fall halite apart. and Calcite thenardite crystals at formFigure a hard,18 show glassy how shell they on are the external surface of a material, while ettringite forms crusts where lime is dissolved and

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formed through efflorescence and subflorescence on the surface as well as in a material’s integrated.voids and The capi drawingsllary pores of halite [23]. and In the thenardite latter case, at Figure the salts 18 show rupture how the they material are formed as the throughcrystallizatio efflorescencen pressure and subflorescence increases. Ettringite on the surface is often as formed well as byin a lime material’s dissolving voids on and the capillarysurface poresof the [ 23material]. In the and latter forming case, thea porous salts rupture layer of the gypsum material crystals as the— crystallizationas this happens, pressure increases. Ettringite is often formed by lime dissolving on the surface of the part of the surface material is dissolved and replaced with gypsum clusters [70]. Calcite material and forming a porous layer of gypsum crystals—as this happens, part of the can be formed on a surface that is moist over a long period and has access to calcium that surface material is dissolved and replaced with gypsum clusters [70]. Calcite can be formed can recrystallise. This is illustrated here by a lime render with a dense surface, where lime on a surface that is moist over a long period and has access to calcium that can recrystallise. that has gone into solution is transported outwards and ultimately deposited as an in- This is illustrated here by a lime render with a dense surface, where lime that has gone soluble, glassy crystal film. into solution is transported outwards and ultimately deposited as an insoluble, glassy crystal film.

(a) (b) (c) (d)

FigureFigure 18. 18.The The general general principles principles for for the the appearance appearance of of the the growth growth of crystalsof crystals of differentof different salts salts and and how how they they gradually gradually breakbreak down down the the surface surface of theof the material: material: (a) ( Halite;a) Halite; (b) ( mirabiliteb) mirabilite and and thenardite; thenardite; (c) ( calcite;c) calcite; (d) ( ettringite.d) ettringite.

SomeSome salts salts can can bind bind water water molecules molecules inin thethe saltsalt crystal (hydration). Interestingly Interestingly for forbrick brick masonry, masonry, the the two two different different sodium sodium sulphates sulphates are are stable stable in an in indoor an indoor climate; climate; mira- mirabilitebilite (Na (Na2SO2SO4 · 10H4 · 10H2O)2 isO) a is hydrated a hydrated salt, salt, as opposed as opposed to thenardite to thenardite (Na (Na2SO2SO4) which4) which is an is ananhydrous anhydrous salt salt [17,21] [17,.21 Mirabilite]. Mirabilite holds holds ten tenwater water molecules molecules bound bound in each in each crystal crystal com- comparedpared to tothe the anhydrous anhydrous salt salt which which ha hass no no bound bound water water at at all. all. This This greatly greatly influences influences the thevolume volume of of the the salt salt crystals crystals and and the the crystallizatio crystallizationn pressure pressure [16,39] [16,39.]. WhenWhen salts salts are are in solutionin solution in masonry, in masonry, they they can affectcan affect the pH the and pH thereby and thereby the solubility the solu- ofbility CaCO of3 throughCaCO3 through an acid-base an acid reaction.-base reaction. They do notThey otherwise do not otherwise cause great cause mechanical great me- damagechanical inside damage the masonry. inside the By masonry. contrast, By these contrast, salts can these cause salts severe can damagecause severe when damage they formwhen crystals, they form and especiallycrystals, and when especially these occur when repeatedly these occur in crystallizationrepeatedly in c cyclesrystallizatio near n thecycles surface near [10 the]. This surface is particularly [10]. This evident is particularly with sodium evident sulphate, with sodium which shifts sulphate, between which itsshifts two differentbetween phases,its two different where one phases, crystal where is much one larger crystal because is much of larger its water because content. of its Slowwater growth content. of mirabiliteSlow growth from of solutionmirabilite produces from solution large produces hydrated large crystals. hydrated After crystals. some dehydrationAfter some and dehydration hydration, and the hydration, more sparingly the more soluble sparingly thenardite soluble crystals thenardite can form crystals a nucleuscan form from a n whichucleus the from mirabilite which the crystals mirabilite can grow crystals [21]. can grow [21]. WhenWhen a saline a saline solution solution dries dries below below its critical its critical RH, salt RH, crystals salt crystalsare formed. are The formed. largest The crystalslargest are crystals formed are at formed just below at just the below critical the RH critical of the saltRH [of23 the]. Different salt [23]. salts Different grow insalts differentgrow in ways. different When ways. sodium When chloride sodium (NaCl) chloride forms (NaCl) halite, forms the crystalshalite, the grow crystals in the grow form in ofthe cube-shaped form of cube crystals-shaped inside crystals a drop inside of water a drop or of larger water crystals or larger are crystals formed are in the formed zone in betweenthe zone air between and water air [10 and]. When water they [10] occur. When in they masonry, occur this in meansmasonry, that this they means have usuallythat they grownhave fromusually the surfacegrown throughfrom the efflorescence. surface through With efflorescence. sodium sulphate, With the sodium crystals sulphate, grow into the thecrystals saline solution,grow into forming the saline fast-growing, solution, forming prism-shaped fast-growi crystals.ng, prism This causes-shaped them crystals. to form This insidecauses fluid-filled them to poresform inside by subflorescence, fluid-filled pores i.e., crystal by subflorescence, formation inside i.e., the crystal material formation [10,14 ].in- Sodium sulphate is the salt that can individually cause the most damage through side the material [10,14]. crystallization in masonry, which is due to its two stable crystal phases; hydrated and Sodium sulphate is the salt that can individually cause the most damage through anhydrous. These two crystal forms at different RH and temperature in accordance with a crystallization in masonry, which is due to its two stable crystal phases; hydrated and phase diagram. Likewise, in repeated cycles of crystallization, it can give rise to crystal anhydrous. These two crystal forms at different RH and temperature in accordance with growth that causes high pressure [21]. With saturated salt solutions where thenardite and a phase diagram. Likewise, in repeated cycles of crystallization, it can give rise to crystal mirabilite coexist, crystallization pressure can reach levels of about 30-60 MPa [19], which growth that causes high pressure [21]. With saturated salt solutions where thenardite and far exceeds the tensile strength of both granite (about 4.8 MPa), brick (approx. 2.8 MPa)

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mirabilite coexist, crystallization pressure can reach levels of about 30-60 MPa [19], which far exceeds the tensile strength of both granite (about 4.8 MPa), brick (approx. 2.8 MPa) and andlime lime mortar mortar (about (about 2.1 2.1MPa) MPa) [63] [63. During]. During hydration, hydration, then thenarditeardite expands expands in in volume volume by by 320320%% [69] [69].. C Crystallizationrystallization behaviourbehaviour in in porous porous materials materials has has been been clearly clearly and and pedagogically peda- gogicallyillustrated illustrated by Pühringer by Pühringer [10] and [10] Zehnder and Zehnder [23]. [23]. A phaseA phase diagram diagram for sodium for sodium sulphate sulphate [19] [shows19] shows how how the thesalt saltmoves moves between between its its differentdifferent phases phases depending depending on relative on relative humidity humidity and andtemperature temperature changes. changes. Every Every time time the climatethe climate in the in material the material passes passes a phase a phase a change a change occurs, occurs, causing causing what what is known is known as a as a crystallizatiocrystallizationn cycle, cycle, and and the the salt salt changes changes its its crystal crystal phase. phase. As As a a result, thethe crystalscrystals degradede- gradethe the material material with with every every crystallization crystallizatio cycle.n cycle. An illustrativeAn illustrative phase phase diagram diagram can be can found be in foundthe in references the references [19,71 [19,71]]. It is. here It is exemplifiedhere exemplified in Figure in Figure 19, where 19, where temperature temperature and RH and were RH weremeasured measured on an on interior an interior surface surface of a church of a church with saltwith crystallization salt crystallizatio problems.n problems.

FigureFigure 19. A 19. diagramA diagram of climate of climate measurement measurement—indoor—indoor temperature temperature [°C] and [◦ C]relative and relativehumidity humidity [%]—[%]—inin a salt a-damaged salt-damaged church church on the on island the islandof Gotland, of Gotland, Sweden. Sweden. The indoor The climate indoor often climate exceeds often ex- the criticalceeds theRH criticalof sodium RH chloride of sodium (NaCl) chloride or is between (NaCl) or the is two between crystal the phases two crystal of sodium phases sulphate of sodium (NaSO4). Illustration by Magnus Wessberg, Uppsala University, and reproduced with his kind sulphate (NaSO ). Illustration by Magnus Wessberg, Uppsala University, and reproduced with his permission. 4 kind permission.

WithWith sodium sodium chloride, chloride, for example, for example, one onecan clearly can clearly identify identify a critical a critical RH for RH c forrystal- crystal- lizatiolization.n. However, However, because because sodium sodium sulphate sulphate can can switch between between different different phases, phases, no no single singlecritical critical RH RH can can be be identified identified for for this this salt salt [16 [16]]. . ControllingControlling the c therystallizatio crystallizationn process process of salts of saltsby controlling by controlling the (micro)climate the (micro)climate in a in walla is wall a possible is a possible option, option, but because but because sodium sodium sulpha sulphatete can be candissolved be dissolved all the alltime, the at time, variousat various RH values RH and values temperatures, and temperatures, an indoor an climate indoor with climate an RH with above an RH93–95 above% would 93–95% be requiredwould be if requiredthe indoor if thetemperature indoor temperature was between was 15 between–20 °C. 15–20Unfortunately,◦C. Unfortunately, this is an this inappropriateis an inappropriate climate that climate can cause that canother cause damage other in damagethe form in of the degradation form of degradation of materi- of als, healthmaterials, problems health for problems occupants for occupants and, in particular, and, in particular, (micro)biological (micro)biological activity [72] activity. The[ 72]. greatestThe greatestdamage damage is caused is causedwhere wherethe climate the climate frequently frequently shifts shiftswithin within the phase the phase diagram diagram so thatso thatmirabilite mirabilite turns turns to tothenardite thenardite and and back back again, again, which which is is why why it wouldwould bebe appropriateappro- priateto avoidto avoid this this specific specific climate. climate. The The thenardite thenardite crystals crystals act act as aas “breeding a “breeding ground” ground for” the for themirabilite mirabilite crystals crystals and andthe total the total volume, volume, and consequently and consequently the crystallization the crystallizatio pressure,n pressure,becomes becomes substantial substantial [20]. [20]. WhenWhen it comes it comes to the to thepresence presence of sodium of sodium sulphate sulphate in brick in brick masonry, masonry, many many case case studiesstudies [4–6] [4 have–6] have clearly clearly shown shown that thatproblems problems are most are most severe severe in façades in façades facing facing sun, sun, rain rain and andwind. wind. This This can canbe berelated related to toareas areas wher wheree temperature temperature and and moisture moisture movements movements at the the surface cause cause most most of of the the micro-cracks micro-cracks between between joints joints and and bricks, bricks, where where moisture moisture loads loadsthrough through (wind-driven) (wind-driven) rain rain are are greatest greatest and and where where the the microclimate microclimate on externalon external as well as as wellinternal as internal walls walls is affected is affected by the by sun the and sun its and radiant its radiant heat. In heat. the northernIn the northern hemisphere, hemi- walls sphere,facing walls south facing and west south are and considerably west are considerably more vulnerable more to weatheringvulnerable fromto weathering salt and frost fromdamage salt and than frost walls damage facing than east walls and facing north, east see Figureand north, 20. see Figure 20.

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(a) (b) (c)

Figure 20. a c . (Figure– ) Examples 20. (a–c) of. Examples salt and frostof salt damage and frost on damage Swedish on neo-Gothic Swedish masonries.neo-Gothic Themasonries. extent ofThe the extent damage of the depends damage depends on a range ofon factors a range including of factors exposure including to exposure the sun, wind-driven to the sun, wind rain- anddriven the rain internal and the and internal external and microclimate. external microclimate.

3.7. The Moisture3.7. The Transport Moisture Phenomenon Transport Phenomenon and Degradation and Degradation of The Masonry of The Core Masonry Core In the typicalIn the problematic typical problematic neo-Gothic neo brick-Gothic walls, brick moisture walls, can moisture be absorbed can be inabsorbed three in three main ways;main by capillary ways; by suction capillary in joints, suction by capillaryin joints, suctionby capillary through suction weathered through bricks weathered and bricks by cracks andand cavitiesby cracks between and cavities bricks between and joints. bricks Problems and jo withints. roofProblems and sheet withcoverings roof and sheet cov- can also affecterings the can ability also affect of water the toability enter of the water masonry to enter walls. the masonry Where the walls. masonry Where core the masonry consists ofcore bricks consists and lime of mortarsbricks and that lime are more mortars porous that than are themore façade porous materials, than the water façade can materials, be transportedwater into can the be masonrytransported core into from the the mason façadery shellcore byfrom means the façade of capillary shell suction.by means of capil- The waterlary suction. can evaporate by diffusion and capillary transport to the surface. Water can dry out both inwardsThe water and outwards.can evaporate If the by masonry diffusion core and is madecapillary of verytransport porous to materials the surface. Water as in the wallscan studied, dry out a both great inwards deal of water and canoutwards. accumulate If the in masonry the masonry core and is it made is difficult of very porous for the moisturematerials to dryas in up the as walls the surfacestudied, tends a great to deal let in of more water water can accumulate than can evaporate. in the masonry and When the waterit is difficult is transported for the moisture towards theto dry interior up as wall the surface surface, tends the salts to let dissolved in more fromwater than can the bricksevaporate. are transported When and the deposited water is transportedwhen the climate towards is right the interior for the formation wall surface, of the salts crystals. Thisdissolved results from in a discoloured the bricks are and transported damaged façade and deposited surface. Thewhen use the of climate dense paint is right for the layers, asphaltformation coatings of crystals. or impermeable This results plasters in a discoloured and renders and can damaged further exacerbate façade surface. the The use situation soof thatdense moisture paint layers, becomes asphalt trapped coatings inside or the impermeable masonry. Withplasters masonry and renders cores, itcan further is interestingexac toerbate note that the thesituation lime in so the that mortar moisture acts as becomes a poorly trapped soluble inside salt; see the Table masonry.1; With CaCO3. Amasonry humid environment cores, it is interesting (RH higher to thannote 100%)that the and lime a lengthyin the mortar period acts of exposure as a poorly soluble are requiredsalt; in see order Table for it1; toCaCO dissolve3. A humid [18]. In environment addition, if there(RH ishigher a type than of salt100 present%) and a lengthy that lowersperiod the pH, of itsexposure solubility are increases required through in order an for acid-base it to dissolve reaction. [18] Inside. In addition, masonries, if there is a there is usuallytype of both salt exposure present that to high lowers relative the pH, humidity its solubility over long increases periods through and a presence an acid-base reac- of salts—thus,tion. the Inside lime masonries, in the mortar there can isdissolve, usually both pulverising exposure the to mortar high relative and reducing humidity its over long load-bearingperiods capacity. and a presence of salts—thus, the lime in the mortar can dissolve, pulverising the Duringmortar the 20th and century, reducing most its load extensive-bearing masonry capacity. repairs have been carried out using cement-based mortar,During regardless the 20th century, of whether most the extensive original masonry masonry was repairs built have with somebeen typecarried out us- of lime mortaring (whichcement can-based be assumed mortar, to regardless be the case of for whether most buildings the original built masonryin Scandinavia was built with before thesome 1940s, type though of lime with mortar significant (which variation). can be assumed It was to probably be the case thought for most that buildings it was built in importantScandinavia to do the repair before using the a1940s, strong though and dense with materialsignificant (cement) variation). and It there was wasprobably no thought real awarenessthat it of was the possibleimportant negative to do the consequences repair using for athe strong masonry and dense wall. Formaterial the brick (cement) and masonry buildingsthere was studied no real [awareness4–6], the archive of the materialpossible showsnegative that, consequences at certain times, for the people masonry wall. have been awareFor the of brick this and masonry have tried buildings to retain studied the use [4– of6] lime, the mortar,archive and material both cementshows that, and at certain lime-basedtimes, mortars people were have used been during aware the secondof this and half have of the tried 20th to century. retain the In large use of masonry lime mortar, and walls thereboth are alwayscement some and moisturelime-based and mortars temperature-related were used during movements the second [63], half and whenof the 20th cen- they are repairedtury. In with large a hard, masonry stiff walls mortar there such a asre cementalways mortar, some moisture lime cement and mortartemperature or -related strong hydraulicmovements lime mortar,[63], and cracks when in they the are surface repaired will eventuallywith a hard, develop. stiff mortar Water such will as cement be able tomortar, enter the lim masonrye cement through mortar theseor strong cracks hydraulic and through lime weatheredmortar, cracks brick. in Ifthe the surface will masonry has a dense surface layer, it may mean that evaporation does not take place at the eventually develop. Water will be able to enter the masonry through these cracks and

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through weathered brick. If the masonry has a dense surface layer, it may mean that sameevaporation rate as humidification, does not take place causing at the moisture same rate to accumulate as humidification, inside the causing masonry moisture behind to theaccumulate dense surface. inside Moreover, the masonry if the behind façade the bricks dense are surface. not well Moreover, connected if to the the façade core of bricks the wallare andnot arewell partly connected freestanding, to the core no capillaryof the wall transport and are of partly water freestanding, can take place no and capillary there willtransport only be of very water limited can take drying place of and the core.there will only be very limited drying of the core. WhenWhen degradation degradation has has been been ongoing ongoing over over several several decades, decades, which which is currently is currently the case the withcase most with large most historic large historic masonry masonry constructions constructions in Scandinavia, in Scandinavia, it has often it has caused often the caused lime binder,the lime initially binder, present initially as binderpresent in as joints binder and in bedding joints and mortars, bedding to leachmortars, out [to4, 56leach]. The out high[4,56] content. The high of moisture content insideof moisture the masonry inside the has masonry likewise has led likewise to frost damageled to frost inside damage the wall.inside What the remainswall. What inside remains the masonry inside the is notmason a beddingry is not mortar a bedding with mortar bonding with ability bonding but mainlyability moist,but mainly lime-containing moist, lime sand.-containing This phenomenon sand. This phenomenon is described inis archivedescribed material in archive [5] wherematerial it was [5]observed where it thatwas both observed the mortar that both and thethe bricks mortar had and been the completely bricks had pulverised been com- andpletely had apulverised high moisture and content—inhad a high moisture older Swedish content literature—in older this Swedish is sometimes literature described this is assometimes “masonry described rot” [73,74 as]; see“masonry Figure 21rot.” [73,74]; see Figure 21.

(a) (b)

FigureFigure 21. 21.(a ()a Neo-Gothic) Neo-Gothic masonry masonry where where accumulated accumulated moisture moisture inside inside the the masonry masonry has has caused caused “masonry “masonry rot”; rot” (;b ()b the) the bricksbricks have have frozen frozen and and split, split, while while the the lime lime mortar mortar has has assumed assumed the the character character and and characteristics characteristics of of wet wet sand. sand.

ThisThis phenomenon phenomenon may may eventually eventually lead lead to to structural structural problems problems [18 [18]]. During. During the the reno- ren- vation of one church tower in 1992, the masons noted that the entire interior of the masonry ovation of one church tower in 1992, the masons noted that the entire interior of the ma- was frozen and pulverised [5]. Where the façade bricks had been laid with insufficient sonry was frozen and pulverised [5]. Where the façade bricks had been laid with insuffi- cohesion (with larger bricks in every seventh course instead of every other course), these cient cohesion (with larger bricks in every seventh course instead of every other course), larger façade bricks had frozen off, see Figure 22. The consequence was that, in addition to these larger façade bricks had frozen off, see Figure 22. The consequence was that, in the façade layer becoming detached from the masonry core, loose material inside the wall addition to the façade layer becoming detached from the masonry core, loose material had fallen off behind the façade layer and created an even larger cavity [5]; see illustration inside the wall had fallen off behind the façade layer and created an even larger cavity in Figure 22. After years of weathering, the surface layers had been damaged, both inter- [5]; see illustration in Figure 22. After years of weathering, the surface layers had been nally (salt crystals) and externally (salt and ice crystals). Mortar of lime and the wall core of damaged, both internally (salt crystals) and externally (salt and ice crystals). Mortar of bricks had been frost-damaged and the lime mortar had also leached and lost its adhesion. lime and the wall core of bricks had been frost-damaged and the lime mortar had also The outer layer of the façade brick had lost its cohesion since these bricks had spalled off leached and lost its adhesion. The outer layer of the façade brick had lost its cohesion and weathering products had increased the volume of material behind the façade layer, movingsince these it outwards bricks had and spalled leaving off it unable and weathering to return to products its original had position. increased When the thevolume outer of layermaterial had lostbehind its adhesion the façade to thelayer, substrate, moving dehydration it outwards towardsand leaving the coreit unable masonry to return could to no its longeroriginal take position. place and When the denserthe outer surface layer layershad lost thereby its adhesion delayed to diffusion, the substrate, so an dehydration increasing amounttowards of thewater core accumulated masonry could in the nomasonry. longer take place and the denser surface layers thereby delayed diffusion, so an increasing amount of water accumulated in the mason- ry.

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Core of masonry bricks, partly frost-damaged and pulverised.

Salt crystals have lifted the linseed oil paint Lean bedding mortar of lime and layer from the plaster. sand, moist and weathered with partially dissolved lime.

Façade bricks with some Internal lime plaster ca weathered surfaces and some 15-30 mm, partly broken binding stones. The weathered by salts. façade brick originally had a hard-burned low permeable surface. The façade layer is now freestanding from the core of the wall in some areas.

Pointing mortar of lime cement with lost cohesion due to thermal and moisture expansion. Water can penetrate into the wall through thin cracks by capillary transport. The low vapour permeability of the pointing mortar prevents moisture from evaporating.

Figure 22. FigureSchematic 22. Schematic illustration illustration of damage of mechanisms damage mechanisms in a solid in masonry a solid wallmasonry with wall a brick with façade. a brick façade.

4. Case Studies4. Case UsingStudies Hemp-Lime Using Hemp as-Lime Sacrificial as Sacrificial Plaster Plaster Hemp-limeHemp is- alime building is a building material material that is a that combination is a combination of hemp of shiv hemp (the shiv woody (the woody core partscore of theparts hemp of the stem) hemp and stem) building and building limes. Hemp-lime limes. Hemp has-lime been has used been as aused building as a building materialmaterial for new for walls new but walls also but as insulation also as insulation material material when renovating when renovating or retrofitting or retrofitting historic buildings.historic buildings. While the While material the material has good has insulating good insulating properties properties [75–78], [75 it is–78] also, it is also moisturemoisture diffusion diffusion open, allowing open, allowing moisture moisture transport transport through through the material. the material. Hemp-lime Hemp-lime has a complexhas a porosity; complex a porosity; micro porosity a micro inside porosity the lime inside binder, the mesolime binder, porosity meso in the porosity hemp in the shiv andhemp lime bindershiv and and lime a macro binder porosity and a macro in the porosity material in due the to material the arrangement due to the of arrangement the hemp shivof the [79 ,80hemp]. Furthermore shiv [79,80]. hemp-lime Furthermore works hemp well-lime as works a basis well for theas a application basis for the of applica- lime render.tion Theseof lime characteristics render. These may characteristics be beneficial may when be using beneficial hemp-lime when asusing a sacrificial hemp-lime as a plaster onsacrificial the internal plaster surface on the of brickinternal masonry. surface of brick masonry. A case studyA case and study a laboratory and a laboratory study were study undertaken. were undertaken.

4.1. ÖRgryte4.1.Ö. New Rgryte Church New Church This brickThis church brick haschurch salt-related has salt- problemsrelated problems which occurredwhich occurred only some only years some afteryears after it it was built.was built. The churchThe church was was built built in the in the 1890s 1890s and and designed designed by by the the architect architect Adrian Adrian Peter- Peterson,son, who who built built over over 30 similar30 similar churches churches [2,3 [2,3]]. Previous. Previous tests tests with with sacrificial sacrificial plasters plasters were were unsatisfactory;unsatisfactory; immediately immediately after after plastering, plastering, there there were were salt depositssalt deposits in the in form the form of of ef- efflorescenceflorescence and subflorescence, and subflorescence, and soon and after soon application after application the plaster the plaster partially partially spalled spalled off off fromfrom the wall. the The wall. salt The causing salt causing most damage most damage has been has analysed been analysed with SEM/EDS with SEM/EDS and and identifiedidentified as sodium as sulphatesodium sulphate [4]. The other[4]. The salts other described salts described above are above also presentare also inpresent the in the buildingbuilding but not togetherbut not together with the with sodium the sulphatesodium sulphate on internal on plasterinternal surfaces. plaster surfaces.

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In 2016, three new test surfaces of different sacrificial plasters were applied to the internal surface of a brick wall in Ö rgryte new church [57]. One of these test surfaces was constructed using a layer of 20 mm hemp-lime (1:3 lime:hemp shiv) with a surface finish of 20 mm lime plaster (NHL 3.5). After construction, the surface was worked with a wooden trowel; see Figure 23.

4.2. Test Lab in Lund A south-facing, brick masonry wall measuring 102 × 212 cm was constructed in 2017

Heritage 2021, 4 as an external wall in the test lab at the Department of Building Materials, Lund364 Faculty of Engineering; see Figure 23. Prior to construction, the bricks were immersed in a solu- tion of water saturated with sodium sulphate. Internally, a lime surface finish was ap- plied to the brick. Then, a layer of 100 mm hemp-lime was applied to the wall, with an In 2016,internal three lime new plaster. test surfaces of different sacrificial plasters were applied to the internal surfaceMaterial of a brick from wall the hemp in Örgryte-lime newat the church test lab [57 in]. Lund One of was these sent test to surfacesPetrolab wasin Redruth, constructedUK using for production a layer of 20 of mm thin hemp-lime section analysis (1:3 lime:hemp samples. shiv) These with were a surface studied finish and of photo- 20 mm limegraphed plaster in (NHLa Brunel 3.5). SP1500 After- construction,XP polarisation the surfacemicroscope. was workedNo salts with were a woodendetected in the trowel; seesamples. Figure After 23. 3.5 years, no salts were visible on the surface of the hemp-lime.

(c) (a) (b)

Figure 23.FigureHemp-lime 23. Hemp case-lime studies: case (studies:a) Hemp-lime (a) Hemp test-lime surface test in surface Örgryte in newÖ rgryte church, new Gothenburg;church, Gothenburg; (b) test wall (b) test of brick wall of brick with sodiumwith sulphate sodium atsulphate Lund University; at Lund University; (c) section ( ofc) section the test of wall: the 1.test Lime wall: plaster, 1. Lime 2. Hemp-limeplaster, 2. Hemp 100 mm,-lime 3. 100 Lime mm, finish, 3. Lime fin- 4. Brick saturatedish, 4. Brick with saturated sodium with sulphate. sodium sulphate.

5. Discussion—What Can Be Done to Prevent Salt-Related Damage in Masonry? 4.2. Test Lab in Lund Removing salts present in historic masonry walls is almost impossible. For thick A south-facing, brick masonry wall measuring 102 × 212 cm was constructed in 2017 masonry such as in basement vaults or high church towers, the salts are present to such as an external wall in the test lab at the Department of Building Materials, Lund Faculty of an extent and at such depth that they are not accessible to desalination at the surface [7]. Engineering; see Figure 23. Prior to construction, the bricks were immersed in a solution of However, depending on which salts occur as well as in the form in which they are pre- water saturated with sodium sulphate. Internally, a lime surface finish was applied to the sent in the masonry, the damage can be minimised by taking the following measures: brick. Then, a layer of 100 mm hemp-lime was applied to the wall, with an internal lime  plaster. Minimising water supply from the outside; Material Adapting from the hemp-limeinternal surface at the layers test lab so in that Lund moistu wasre sent transport to Petrolab canin be Redruth, controlled and UK for productiondrying of can thin take section place analysis; samples. These were studied and photographed in a Brunel SP1500-XPApplying climate polarisation control microscope. measures (primarily No salts wereindoors detected); in the samples. After 3,5 years,The no climate salts were envelope visible must on the of course surface have of the a functioning hemp-lime. water run-off and drainage. Outer walls should not be able to absorb more water than can be evaporated. If there 5. Discussion—Whatare points or Cancracks Be in Done the to masonry Prevent that Salt-Related allow water Damage to enter in Masonry? the wall, the masonry Removingshould be salts able present to support in historic moisture masonry transport walls towards is almost the outside impossible. so that For drying thick can take masonryplace, such i.e., asin its basement surface must vaults not or be high too churchdense if towers, there ar thee portions salts are of present brick with to such high capil- an extentlary and suction at such capacity depth that behind they arethe not dense accessible surface. to In desalination older masonry, at the there surface is [ a7 ]. constant However, depending on which salts occur as well as in the form in which they are present in the masonry, the damage can be minimised by taking the following measures: • Minimising water supply from the outside;

• Adapting internal surface layers so that moisture transport can be controlled and drying can take place; • Applying climate control measures (primarily indoors); • The climate envelope must of course have a functioning water run-off and drainage. Outer walls should not be able to absorb more water than can be evaporated. If there are points or cracks in the masonry that allow water to enter the wall, the masonry should be able to support moisture transport towards the outside so that drying can take place, i.e., its surface must not be too dense if there are portions of brick with high capillary suction capacity behind the dense surface. In older masonry, there is a constant ageing process, Heritage 2021, 4 365

which means that all masonry takes in some water by absorption through vertical wall surfaces such as bricks and joints. Indoor surfaces should not be too dense so that dehydration can take place. If de- hydration is difficult or even impossible, salts are deposited under the surface layer and “eat” into the material underneath. Two examples of documented renovation measures that caused more harm than good are: 1. Repair of lime plaster with a dense plaster based on cement; 2. Painting with latex paint on a lime plaster [4]. If a sacrificial coating of lime plaster is applied on the internal surface of a wall, it needs to have a pore structure that allows it to absorb moisture from the substrate by capillary suction while not containing excessive cavities. There is no clear research as to what such a plaster should look like in detail. If the plaster consists of a lean lime and is porous with a tightly worked surface, it can be subjected to extensive damage [4,69]. Where that happens, the binder is not able to withstand the crystallization pressure formed when salts are deposited in the pores. Rodriguez-Navarro and Doehne [14] state that in materials containing only a few large voids together with a limited amount of capillary pores (a limited inner surface area), the salt solution can reach the surface more quickly and form efflorescence rather than subflorescence. They have also stated that with pore sizes below 100 nm (thenardite; 5.2 nm for halite), the crystallization pressure will often exceed 3 MPa, which is more than the tensile strength limit values of most inorganic building materials [17]. If the surface layer contains extremely small pores, halite (NaCl) crystals can cause damage through subflorescence. These are the factors that must be taken into account when undertaking repairs or applying sacrificial or new paint layers to a surface. Mortars must therefore be prepared, applied and processed so that they have a specific pore structure [69]. In addition, different measures are needed depending on the salt that has caused the damage. For chlorides, renovation materials can be chosen whereby salts can be deposited on the surface without causing damage that may be too aesthetically disturbing, such as paint loss. For sulphates, a measure can be chosen whereby salts are deposited inside an inner substrate behind the plaster. This can accommodate the salt crystals and withstand its crystallization pressure without allowing the salts to damage the surface plaster. Where salts occur indoors or in a plaster, it is possible in some cases to control the climate so as to minimise cycles of crystallization. This is theoretically only possible for certain salts and certain combinations of salts. It is, therefore, first and foremost important to identify which salts are present in the masonry and to identify their critical RH to determine their point of crystallization. If several salts are present, a range of critical RH needs to be calculated [22]. The climate can then be controlled for different spaces so that the changes in RH do not repeatedly pass critical levels. However, external factors can also affect the indoor climate in certain types of spaces. For example, solar radiation can influence both the indoor climate and masonry surfaces, thereby creating local variations in the wall’s microclimate. When sodium sulphate is present, however, it is difficult, if not impossible, to create an optimal climate to avoid crystallization. This is because it starts to crystallise at very low RH and varies in crystallization phases up to 93% RH at 20 ◦C[16]. Here, it is important that additional moisture loads on the façade are minimised and evaporation is possible on both the external and the internal surfaces of the masonry.

Results and Discussion of the Hemp-Lime Case Studies The test wall at Örgryte new church was evaluated regularly over four years. An ocular inspection with macro photography showed no signs of salt efflorescence on the test surface. No salts were visible on the surface of the hemp-lime test wall, whereas salts were observed already after a few months on the surface of one of the other test surfaces, with a sacrificial layer of clay mortars behind the lime plaster. HeritageHeritage2021 2021, 4, 4 FOR PEER REVIEW 36618

AfterAfter one one year, year, severe severe damage damage was was observed observed on on the the external external surface surface of theof the test test wall wall at Lundat Lund Faculty Faculty of Engineering, of Engineering, with with some some extensive extensive scaling scaling caused caused by the by saltsthe salts inside inside the brick.the brick. However, However, an ocular an ocular inspection inspection with macro with photographymacro photography of the internal of the internal wall surface wall showedsurface no showed signs of no salt signs efflorescence of salt efflorescence after 3,5 years. after The 3. thin5 years section. The analysis thin section clearly analysis shows thatclearly the shows lime encloses that the all lime the encloses hemp shiv all inthe the hemp material. shiv in In the addition, material. two In types addition, of micro two poretypes structures of micro canpore be structures found; one can in be the found; lime plasterone in andthe lime another plaster in the and hemp another shiv. inNo the signshemp of shiv. weathering No signs or of degradation weathering causedor degradation by the sodium caused sulphate by the sodium could besulphate observed could in thebe observed thin section in samples,the thin section see Figure samples, 24. see Figure 24.

FigureFigure 24.24. HempHemp-lime-lime from from 115 115–130–130 mm mm inside inside the the wall wall closest closest to tothe the brick brick photographed photographed in a inpo- a larisation microscope. A homogenous lime (K) encloses the hemp shiv (H). In the lime, there are polarisation microscope. A homogenous lime (K) encloses the hemp shiv (H). In the lime, there round air pores of various sizes (P), mostly without capillary pores. In the hemp shiv, the fibre di- are round air pores of various sizes (P), mostly without capillary pores. In the hemp shiv, the fibre rection can be observed as well as the shiv’s pore structure, which follows the direction of the fi- directionbres. The can photo be observed shows a as2.65 well mm as-wide the shiv’s sample. pore structure, which follows the direction of the fibres. The photo shows a 2.65 mm-wide sample. 6.6. ConcludingConcluding RemarksRemarks InIn orderorder toto bebe ableable toto undertakeundertake aa durabledurable renovation,renovation, itit isis importantimportant toto firstfirst identify identify thethe naturenature ofof the salt salt damage damage and and the the type type of of salt salt that that is present is present in the in themaso masonry.nry. The Thenext nextstep stepis to isidentify to identify the factors the factors affecting affecting the occurrence the occurrence of the of salt, the salt,the salt the transport, salt transport, crys- crystallizationtallization patterns patterns and and crystallizatio crystallizationn cycles cycles based based on onthe the microclimate microclimate at atthe the origin origin of ofthe the damage. damage. Certain Certain factors factors can can then then be be addressed through through continuouscontinuous maintenance. maintenance. TheseThese are are as as follows: follows:  The masonry’s behaviour in terms of absorption and evaporation, both externally • The masonry’s behaviour in terms of absorption and evaporation, both externally andand internally; internally;  • ThermalThermal andand moisturemoisture movements, movements, adhesionadhesion andand compatibility compatibility between between bricks bricks and and pointingpointing mortars; mortars; • PorePore structure structure and and frost frost resistance resistance of of porous porous materials; materials; • MinimisationMinimisation of of crystallization crystallizatio cyclesn cycles on and on near and wall near surfaces wall surfaces through throughclimate controlclimate. control. By creating the conditions for an altered microclimate that can be obtained by intro- By creating the conditions for an altered microclimate that can be obtained by in- ducing a heat insulating layer and by ensuring a substrate behind the plaster has a proper troducing a heat insulating layer and by ensuring a substrate behind the plaster has a pore structure, salt transport to the surface can be slightly reduced. At the same time, proper pore structure, salt transport to the surface can be slightly reduced. At the same the brickwork must be intact in order to reduce water absorption and thus the need for time, the brickwork must be intact in order to reduce water absorption and thus the need evaporation inwards. for evaporation inwards. The exterior brickwork materials need to be appropriate and compatible with each otherThe as well exteri asor compatible brickwork with materials the original need masonryto be appropriate material. Theyand compatible need a suitable with poreeach structureother as well for bothas compatible moisture with transport the original and frost masonry resistance, material. and They they need a moisture suitable andpore temperaturestructure for expansion both moisture capabilities transport that and can frost prevent resistance, cracking. and Research they need issues moisture relating and to thetemperat compatibilityure expansion of repair capabilities materials that for can neo-Gothic prevent brickcracking. buildings Research need issues to be relating studied to morethe compatibility closely in the of future. repair materials for neo-Gothic brick buildings need to be studied moreSalts closely deposited in the future. internally on wall surfaces move by means of moisture transport into the masonry.Salts deposited As the masonry internally can on absorb wall surfaces a lot of moisture,move by themeans salts of dissolve moisture and transport move with into thethe drying masonry. water As towards the masonry a surface. can absorb Through a temperaturelot of moisture, changes the salts in the dissolve masonry and as move well

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as in the surrounding air in combination with high relative humidity, salts in the masonry will often exceed their crystallization thresholds, with repeated damage as a result. With salts such as sodium sulphate, it is not possible to control the indoor climate in order to completely avoid salt crystallization. Instead, the effect of the damage can be minimised by using appropriate and compatible materials when renovating the walls in question. Maintenance-free materials do not exist, but materials can be more or less complicated to maintain. It is important for maintenance of the brickwork to be easy and manageable, and no measures should aggravate or cause further damage. When choosing brick, mortar, pointing mortar, render and plaster for the renovation of historic masonry walls, it is of the utmost importance to have an understanding of the complex functions of the masonry. The results of the case studies showed that hemp-lime could be a good option for use as a sacrificial plaster on salt-damaged masonry walls. The results from the case study and labo- ratory study to date have led to full-scale-studies of hemp-lime plaster beneath the internal lime plaster on salt-damaged brick masonry walls. A follow-up is planned in 2021–2022. This type of neo-Gothic masonry will continue to have some degree of frost and salt-related moisture problems. Careful maintenance planning covering at least the next 50 years is therefore essential. Above all, these buildings require well-planned continuous maintenance of the masonry with damaged material being replaced with appropriate, compatible materials.

Author Contributions: Conceptualisation, K.B.; methodology, K.B. and P.S.-d.B.; software, K.B.; vali- dation, K.B. and P.S.-d.B.; formal analysis, K.B.; investigation, K.B. and P.S.-d.B.; resources, K.B. and P.S.-d.B.; data curation, K.B. and P.S.-d.B.; writing—original draft preparation, K.B.; writing—review and editing, P.S.-d.B. and K.B.; visualisation, K.B.; project administration, P.S.-d.B.; funding acquisi- tion, P.S.-d.B. and K.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Swedish Energy Agency through the Spara och Bevara research programme, grant number 42308-2. The role of this funding source was strictly financial. During the course of the project, two seminars were held as part of the research programme, where the outline of the project and some preliminary results were presented. Acknowledgments: The case studies that this paper refers to are mainly projects financed by the Swedish Church, and we gratefully acknowledge their help. This work would not have been pos- sible without the help of mason Henrik Nilsson and technical staff Bengt Nilsson (in memoriam) and Stefan Backe. Their help is gratefully acknowledged. Our thanks go also to Petrolab in Re- druth, UK for preparing samples and Jan-Erik Lindqvist for help with the analysis of polarisation microscope results. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study, the collection, analysis, or interpretation of data, the writing of the manuscript or the decision to publish the results.

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